1. Field of the Invention
The present invention relates to a method of etching a magnesium oxide film that is adjacent to a metal layer.
2. Description of the Related Art
With recent improvements in recording density of magnetic recording devices such as magnetic disk drives, there has been a demand for improving the performance of thin-film magnetic heads. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head unit including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head unit including an induction-type electromagnetic transducer for writing are stacked on a substrate. Each thin-film magnetic head has a medium facing surface to face a recording medium.
Examples of MR elements include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect.
Read head units are required to have high sensitivity and high output characteristics. As the read head units that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer opposite from the spacer layer. The free layer is a ferromagnetic layer whose magnetization direction varies in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose magnetization direction is pinned. The antiferromagnetic layer is to pin the magnetization direction of the pinned layer by means of exchange coupling with the pinned layer. For spin-valve GMR elements, the spacer layer is a nonmagnetic conductive layer. For TMR elements, the spacer layer is a tunnel barrier layer. The tunnel barrier layer is typically formed of an insulating material such as aluminum oxide (Al2O3) or magnesium oxide (MgO).
Examples of GMR elements include one having a current-in-plane (CIP) structure in which a current for magnetic signal detection (hereinafter referred to as sense current) is fed in a direction parallel to the planes of the layers constituting the GMR element, and one having a current-perpendicular-to-plane (CPP) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as a direction perpendicular to the planes of the layers constituting the GMR element. TMR elements have the CPP structure.
Now, an example of the configuration of a read head unit incorporating an MR element of the CPP structure will be described. The read head unit includes first and second electrodes located at a distance from each other, and an MR element interposed between the first and second electrodes. The MR element has a first surface in contact with the first electrode, a second surface in contact with the second electrode, a front end face located in the medium facing surface, a rear end face opposite to the front end face, and first and second side surfaces opposite to each other in the track width direction. The first and second electrodes are used for feeding a sense current to the MR element. Each of the first and second electrodes may also function as a shield for the MR element.
The read head unit further includes first and second bias magnetic field applying layers located on opposite sides of the MR element in the track width direction, a first insulating film interposed between the first bias magnetic field applying layer and the first side surface of the MR element, a second insulating film interposed between the second bias magnetic field applying layer and the second side surface of the MR element, and an insulating refill layer disposed around the MR element and the first and second bias magnetic field applying layers.
The first and second bias magnetic field applying layers apply to the MR element a bias magnetic field for orienting the magnetization of the free layer in a predetermined direction when no signal magnetic field is applied to the MR element. The first and second bias magnetic field applying layers include respective metal layers each made of a magnetic alloy. The metal layers are exposed in the respective top surfaces of the first and second bias magnetic field applying layers, for example. The insulating refill layer is in contact with the rear end face of the MR element. The insulating refill layer is made of an insulating material. The read heat unit having such a configuration is disclosed in JP-A-2004-199812, for example.
An example of the manufacturing method for the aforementioned read head unit will now be described. In the manufacturing method, first, an MR film that later becomes the MR element is formed on the first electrode. The MR film is composed of a plurality of layers stacked. The uppermost layer of the MR film is typically a metal layer. Then, the MR film is selectively etched using a first mask to provide the MR film with the aforementioned first and second side surfaces. Next, the first and second insulating films and the first and second bias magnetic field applying layers are formed in succession, and the first mask is removed. The MR film is then selectively etched using a second mask to provide the MR film with the aforementioned rear end face. Next, the insulating refill layer is formed and the second mask is removed. The second electrode is then formed on the MR film.
To manufacture a magnetic head including the read head unit described above, components of a plurality of magnetic heads other than the substrates are formed on a single wafer that includes portions to later become the substrates of the plurality of magnetic heads, whereby a substructure including rows of a plurality of pre-head portions is fabricated. The plurality of pre-head portions become individual magnetic heads later. Then, the plurality of pre-head portions are separated from each other by cutting the substructure, and the cut surfaces are polished to form medium facing surfaces. The formation of the medium facing surfaces provides the MR films with the front end faces, thereby making the MR films into MR elements.
In the process of manufacturing the read head unit described above, the insulating material used to form the first and second insulating films and/or the insulating material used to form the insulating refill layer may deposit on the MR film into an unwanted insulating film. If the unwanted insulating film remains on the MR film, there may occur continuity failure between the MR element and the second electrode. It is thus necessary to remove the unwanted insulating film.
For example, wet etching is employed to remove the unwanted insulating film. The wet etching needs to be performed under such a condition that the metal layers adjacent to the unwanted insulating film, that is, the uppermost metal layer of the MR film and the metal layers of the first and second bias magnetic field applying layers, are not etched at all or are hardly etched.
JP-A-2004-199812 discloses removing an unwanted insulating film of Al2O3 by wet etching using tetramethylammonium hydroxide as the etchant.
Here, a case will be contemplated in which the first and second insulating films are formed of magnesium oxide. In this case, the unwanted insulating film is formed by magnesium oxide. Thus, in such a case, a wet etching method is needed that can selectively etch a magnesium oxide film adjacent to a metal layer. However, such a method has not been known.
The inventors of the present application attempted to etch a magnesium oxide film by using tetramethylammonium hydroxide as the etchant, and found, however, that the magnesium oxide film was hardly etched.